1. Field of the Invention
The present invention relates to a double-sided copper-clad laminate for forming a capacitor layer and a method for the manufacture thereof, and a printed wiring board obtained using the double-sided copper-clad laminate for forming a capacitor layer.
2. Description of the Related Art
In recent years, the clock frequency of computers has reached a GHz level, and the circuit design of central processing units (CPUs), the performance of IC chips, and the like has become important. The circuit design of printed wiring boards used for mounting these has also become very important. In order to cope with the above-described increased signal transmission speed, the manufacturers of printed wiring boards have made various devices such as increase in the number of layers of a printed wiring board from the aspect of structures, and the shortening of signal transmission distance by changing circuit design from the aspect of circuit arrangement.
A capacitor formed on a printed wiring board plays the role of stably supplying the operating power of the device, and has generally disposed on the outer layer of the printed wiring board. However, a method for forming a capacitor on the inner layer of a multi-layer printed wiring board using a double-sided copper-clad laminate or the like has become widely used for the reason that it can be easily thinned, and excellent properties can be achieved. In order to such a capacitor layer, a thin double-sided copper-clad laminate has been used.
The double-sided copper-clad laminate used for forming the capacitor layer normally uses an FR-4 base material produced from an epoxy-resin-impregnated glass cloth an insulator (=dielectric layer), and copper foils are adhered on the both sides thereof At this time, the FR-4 base material as thin as possible has been used to secure a high electric capacity. Capacitor circuit patterns have been formed on the both sides of the double-sided copper-clad laminate of such a structure, and have been used as a material for forming a built-in capacitor layer of the multi-layer copper-clad laminate. In order to obtain a high electric capacity as a capacitor circuit, the manufacture of a double-sided copper-clad laminate having further thin dielectric layer has been attempted (Patent Document 1: Japanese Patent Laid-Open No. 2002-9416; Patent Document 2: Japanese Patent Laid-Open No. 2001-339167).
However, an electrodeposited copper foil is generally used as the copper foil composing a thin double-sided copper-clad laminate for forming the capacitor layer, and in addition, nodular treatment to make the surface irregular for obtaining the anchor effect to improve adhesion between the electrodeposited copper foil and the dielectric layer is performed on the joining interface of the electrodeposited copper foil to the dielectric layer.
Therefore, as the thickness of the dielectric layer of the double-sided copper-clad laminate becomes thinner, the nodular-treated surfaces approach to each other, and if there is an abnormally grown nodular-treated area, or an abnormally raised portion is produced on the copper foil before nodular treatment, the copper foil surfaces are partially short-circuited to each other, and when a voltage is impressed between the copper foil surfaces, conduction occurs between facing copper foils, causing a serious problem in thinning the dielectric layer, and significantly lowering the product yield to 40% or below.
As a result, the production costs of the double-sided copper-clad laminate having a thin dielectric layer have inevitably increased, and the supply of inexpensive and high-quality products have become difficult.